METEIOEHYNCHUS. 101 



hidden by the overlap of the bases of the neural arch and by the anterior wedge-bone. 

 At its postero-inferior angle there is a rounded prominence which, with a corresponding 

 projection on the centrum of the axis, bears the articular surface for the head of the 

 second rib. Posteriorly it ends in a nearly flat vertical surface which, in the adult, 

 fuses with the centrum of the axis. 



The atlas of Geosaurus, as described and figured by Fraas *, differs in a remarkable 

 degree from that of Metnorhynchus, the anterior subvertebral wedge-bone having 

 apparently lost its wedge-shape and forming, as it were, the lower half of a short 

 centrum bearing on its sides facets for the first pair of ribs, the odontoid being much 

 reduced. The specimens, however, are described as being unsatisfactorily preserved in 

 this region, so probably the differences are more apparent than real. 



The centrum of the axis (text-fig. 01) resembles that of the succeeding cervicals in 

 its general form, but differs in fusing with the odontoid in front and in bearing at its 

 antero-ventral angle a prominence which helps to form the surface for the head of its 

 rib. In its middle portion the centrum is somewhat constricted, and its ventral surface 

 bears a rounded ridge which is much roughened at its ends. The posterior face is 

 gently concave and nearly circular in outline, there being only a slight flattening 

 beneath the neural canal. The neural arch [ax. a.) is large, and above bears an 

 elongated neural spine which slopes up gradually from its anterior end, the summit 

 being, as a rule, much roughened. Anteriorly, on the sides of the arch are the 

 zygapophysial facets for union with the zygapophyses of the atlas. The posterior 

 zygapophyses {p.z.) are of the normal form; they are oval in outline and project 

 considerably beyond the centrum. The pedicles of the arch unite with the whole 

 length of the centrum, and anteriorly may even extend a little on to the odontoid. 

 Near the base of the sides of the arch is a pair of prominent diapophyses (d.) for the 

 tubercle of the rib, which is here bifurcated. In Steneosaurus this diapophysis is 

 scarcely at all developed, and the only indication of the forking of the rib is a slight 

 prominence on its upper edge (text-fig. 39 B, p. 103). 



In none of the skeletons examined are there more than five cervical vertebra behind 

 the atlas and axis, regarding as cervicals the vertebrae in which the parapophysis is 

 borne entirely on the centrum. If this rule be adopted, then in the Metriorhynchus 

 skeleton described by Arthaberf, the two vertebrae which he calls the first and second 

 dorsals would be cervicals, making seven in all behind the axis. It seems very unlikely 

 that this is correct, and some vertebrae belonging to another individual have probably 

 been included. Nevertheless, it may be pointed out that in Steneosaurus there were at 

 least seven postaxial cervicals, and it is possible that, as Fraas has suggested, the 

 shortening of the neck in Metriorhynchus is one of the consequences of its more 

 extreme adaptation to a purely pelagic life. 



* Palreontographica, vol. xlix. (1902) p. 49, pi. vii. fig. 5. 

 t Beitr. Palaont. Oesterr.-Ung. vol. xix. (1906) p. 294. 

 TART II. . Y 



